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Hi Folks,


My old and trusty 465 was turned on the other day and nothing is displaying on the screen. Not a dot, not a line -- completely blank.


The indicator lights seem to be working, and it seems to find a valid trigger on vertical inputs, so I believe some portion of the circuitry must be ok.


I took it apart last night and did some snooping:


1. All the non-lethal bias voltages seem very good that I tested -- the +55volt, -8 volt, +5 volt, +15 volt (I that's what I tested). No ripple (under 1mV), and dead-accurate.


2. The z-axis amplifier, which I initially suspected, seems very good. The intensity knob can adjust the test point (TP1486) from 15 volts to 41 volts, and 27 does seem about the middle.


Pressing "beam finder" does not find anything.


Here is what I think I should check, and I would appreciate your thoughts and comments:
Vertical and Horizontal deflection plate voltage: If this were too high or too low (i.e., "very negative"), the beam would deflect into the plate or off-screen, correct? What's a reasonable range, differential, between a set of plates? I seem to remember applying 20 volts in my undergraduate physics class to a CRT on a table. CRT Heater voltage: Is there an easy-access test point for this 6.3 volt AC point? Should I try and read the current too? Or maybe ohm it out? Barring the above, time to check the cathode and anode voltage, correct? Thanks very much. The last scope I took apart had tubes in it and was larger than a mini fridge, so I really appreciate any insight you folks might have.


Thanks again,


--Elliott

Hi,


The NO ripple report sounds too good. Can you check the cathode voltage? Usually 2 to 3 kv.


Jerry Massengale

Get the service manual for your scope; there are free versions available online.
Google "tektronix 465 service manual".

The CRT filiment is at -2450V, so you don't want to measure that. On the other
hand, if you do have a HV probe, you do want to check that. If it says 0V,
that's your problem. Don't check it without an appropriate HV probe though.

Check the fuse for the HV inverter power supply. A reasonably common
fault is for Q1418 or C1419 to short and blow fuse F1419. You can find
those on diagram 10, CRT circuit. The transistor is heat sinked to the
rear cabinet and the cap is under the HV shield. It's also possible the
HV multiplier has failed; the output of that is 14-16KV. i

Crank the intensity up in a dark room. If the HV supplies are working and the
beam is off screen, you should see some greenish glow. That likely means
a fault in either the H or V amp and you won't have to mess with the HV
supplies.

--
Paul Amaranth, GCIH | Rochester MI, USA
Aurora Group, Inc. | Security, Systems & Software
paul@... | Unix & Windows

Hi Paul,

I will give your suggestions a try. I actually do have an enormous red high voltage probe although I have not used it in ages. I think it's from Heath Kit. I do have a copy of the service manual although it is a bit difficult to navigate. The schematic was definitely helpful when checking out the z-amplifier last night.


Thanks very much -- looking forward to getting my scope back!


--E

I believe (someone please correct me if wrong) that you can simply
disconnect the deflection wires at the CRT, and of course that should give
you a blinding dot if you have HV, and if everything else is intact.

Also check for a filament glow. No idea if these tubes are prone to it but I
once had a scope with a blank screen because of an open filament.

Dan

On 11 Apr 2016 13:19:10 -0700, you wrote:

...

Vertical and Horizontal deflection plate voltage: If this were too high or too low (i.e., "very negative"), the beam would deflect into the plate or off-screen, correct? What's a reasonable range, differential, between a set of plates? I seem to remember applying 20 volts in my undergraduate physics class to a CRT on a table.

For the horizontal plates it is about +/- 80 volts differential for
full deflection. For the vertical plates it is about +/- 32 volts
differential for full deflection. These numbers can be worked out
from the schematics, control settings, and waveforms shown in the
service manual. Maybe someone can double check me on this.

With no input, a single channel active, and a slow sweep speed, a
voltmeter can be used to verify that the horizontal and vertical CRT
outputs are doing what they are suppose to do.

CRT Heater voltage: Is there an easy-access test point for this 6.3 volt AC point? Should I try and read the current too? Or maybe ohm it out? Barring the above, time to check the cathode and anode voltage, correct? Thanks very much. The last scope I took apart had tubes in it and was larger than a mini fridge, so I really appreciate any insight you folks might have.

The resistance will be the safest thing to measure.

Check fuse F1419 on schematic 10.

If you have a secondary working oscilloscope, the collector of Q1418
on schematic 10 will show if the high voltage inverter is operating.
Another good way to test the inverter is measuring the voltage at the
collector of Q1404; it will be much lower than the indicated +4.4
volts if there is no negative high voltage cathode supply.

I would not bother checking the anode voltage for now since the
oscilloscope will show a recognizable display without it.

Check fuse F1519 near the rear of the scope next to the bridge rectifiers. It's a 1.5A fuse for the +15 unregulated supply going to the transformer for the high voltage multiplier. If it's blown then the tantalum cap C1419 under the high voltage cover may be shorted.

The easiest thing to try is to short the V plates together & the H plates together.Don't trust the position indicators to tell you the whole story; they indicate what the amplifiers are doing, not the CRT.

I would be very nervous applying an ext filament transformer to the CRT.That filament is usually at a high negative voltage, IIRC correctly, on most scopes, it is greater than -1KV.
?HankC, Boston WA1HOS

[Non-text portions of this message have been removed]

Well well well.

You folks had some good ideas.

Here is what I found so far:

Unregulated +15 fuse is good.

Vertical deflection voltage (measured differentially) is -31.5 to +32.0 (seems about right). Left it around zero.

Horizontal deflection voltage (measured differentially) is -10 to +83 (seems wrong). I have always felt like the "center" of my horizontal knob was way off, now I know why. I don't think this is the root cause of the no-trace-at-all problem though. I left the knob at approximately "zero" volts, about where I always had the dial it seems. Something to troubleshoot later. Measured separately, the L wire varied (reference to ground) from 58.1 to 48.3. The R wire varied from 11.8 to 94.8, also referenced to ground. Presumably the circuit driving the L side is the one most out of line, I would think.

Then I got brave and hooked up my Simpson 260 from chassis to the "-2450" test point (TP1423 I believe). On the 5kv range it hardly moved. On the 500 volt scale it went about half way up. On the DMM this measured -490V. Now, it's 11 pm and I know better than to keep at it this evening (as much as I want to). Any pointers? I will review the cathode voltage circuits tomorrow and take another peek tomorrow evening.

Thanks very much again,

--E

On 12 April 2016 at 07:13, w7qed@... [TekScopes] <
TekScopes@...> wrote:

Then I got brave and hooked up my Simpson 260 from chassis to the "-2450"
test point (TP1423 I believe). On the 5kv range it hardly moved. On the 500
volt scale it went about half way up. On the DMM this measured -490V. Now,
it's 11 pm and I know better than to keep at it this evening (as much as I
want to). Any pointers? I will review the cathode voltage circuits tomorrow
and take another peek tomorrow evening.

?What's the impedance of the meter? It's most likely that it's shorting out
the HV?. You need to use a HV probe ideally with gigohm impedance.

Malcolm

Simpson 260 VOM's provide 20 kOhms/volt sensitivity on all DC volts
ranges, as the basic meter movement is 50 microamps. A Simpson 260 with
a 5 kV range therefore provides circuit loading of 100 megohms. Many
(most?) of the 260's do not have a 5 kV range.

Mike N4MWP

On 04/12/2016 06:10 AM, Malcolm Hunter malcolm.r.hunter@...
[TekScopes] wrote:

On 12 April 2016 at 07:13, w7qed@... [TekScopes] <
TekScopes@...> wrote:

Then I got brave and hooked up my Simpson 260 from chassis to the

"-2450"

test point (TP1423 I believe). On the 5kv range it hardly moved. On

the 500

volt scale it went about half way up. On the DMM this measured

-490V. Now,

it's 11 pm and I know better than to keep at it this evening (as

much as I

want to). Any pointers? I will review the cathode voltage circuits

tomorrow

and take another peek tomorrow evening.

?What's the impedance of the meter? It's most likely that it's
shorting out
the HV?. You need to use a HV probe ideally with gigohm impedance.

Malcolm

[Non-text portions of this message have been removed]

[Non-text portions of this message have been removed]

Hi Mike,

Mine does indeed have the 5kv option (aerospace surplus is wonderful...).


Given that I have the same reading on both these meters, I think it is safe to assume the cathode voltage is undesirably low. But I will borrow a proper probe later this week from a friend and make sure.


What do you think the output impedance is of that circuit, and how much current can it handle?


Also, should those neon bulbs be lighting up in the high voltage section? They are not right now, thought I'd ask.


Thanks,


--E

On 12/04/16 17:07, w7qed@... [TekScopes] wrote:

What do you think the output impedance is of that circuit, and how much current can it handle?

I used a 3*33Mohm+100kohm voltage divider to ground. Of course I don't know how much that dragged the cathode voltage down, but I have no reason to suspect it was significant.

No, I don't claim the technique was safe, nor do I recommend it.

Also, should those neon bulbs be lighting up in the high voltage section? They are not right now, thought I'd ask.

They ensure the grid-cathode voltage isn't excessive. On mine they fire when the power is turned off, for several seconds.

There's another neon in the PSU, and it flashes every second or so.

Caution: I've only been fettling 465/485s since February.

The neon's only light if a fault causes excessive grid cathode voltage. They may flash on switch off.

I have measured the cathode voltage on a 454 and on a 485 with a 20k/V meter and both measured within the expected tolerance on a working scope. The 465 may of course be different!

Regards,

Roger

I don't have a manual, nor schematic, for a 465 scope. So I can't make
any guesstimates as to what the "impedance" level is for the circuitry
associated with the high voltage point that you have measured. Also, I
don't know whether any neon bulbs should be lit, or not lit; although I
suspect that any neon lamps associated with the CRT are probably surge
suppression devices and should be unlit under normal operation and
circumstances- although neon lamps in amplifier circuits are a different
matter, as they might be for level shifting (and therefore will be
normally lit). Perhaps other Tekscopes group members can comment.

Mike N4MWP

On 04/12/2016 12:07 PM, w7qed@... [TekScopes] wrote:

Hi Mike,

Mine does indeed have the 5kv option (aerospace surplus is wonderful...).


Given that I have the same reading on both these meters, I think it is
safe to assume the cathode voltage is undesirably low. But I will
borrow a proper probe later this week from a friend and make sure.


What do you think the output impedance is of that circuit, and how
much current can it handle?


Also, should those neon bulbs be lighting up in the high voltage
section? They are not right now, thought I'd ask.


Thanks,


--E

[Non-text portions of this message have been removed]

In the 465 calibration procedure a Simpson 262 (or a Triplett 630-NA) is used as DC voltmeter to check the HV voltage. Both the Simpson 262 and 260 have 20 kOhm/V all range. The 262 has a 4 kV range, so even higher load than the 260 with 5 kV range. Hence the 260 should be OK.

Albert

20 kOhm/V is a measure of the meter's sensitivity.That doesn't necessarily mean it can measure 20Kv. The test leads are probably not insulated for high voltage either.
Most multimeters would need a HV probe to measure above 1 Kv.
I may have a 260 manual so I can look it up if you need me to.
?HankC, Boston WA1HOS?


[Non-text portions of this message have been removed]

Which should be more than enough. My Fluke 6 kilovolt high voltage
probe is only 75 megohms and it works fine for measuring cathode
voltages.

You could verify if the inverter is having a problem supplying current
to the probe by also monitoring the low voltage side at the regulator.
If the Simpson is loading the output down, then the feedback loop will
show this.

Hi Folks,

There definitely is something going on with the inverter on the low-side. I took a short trip down the troubleshooting tree in the service manual and this is what I found:


I can adjust the "HV ADJ" pot, R1400, and indeed the wiper voltage is adjustable from -8 to +5. This has no effect on the -2450 voltage though, it remains at -490.


The transistor connected to this pot (Q1404) seems suspect. At the base I see 3.61 volts, collector is 4.4, and emitter is 3.0. These are all far off from the voltages in the schematic -- although I did not try adjusting the R1400 pot to attain similar voltages. Unfortunately this is one of the few transistors that is soldered in, so I did not yank it out and test it immediately (will do that tonight probably). I also noticed that the +120 volt unregulated DC rail is running pretty rich at +146 volts. I first noticed this at R1402, the resistor that forms part of the adjustable network at the base of Q1404. The voltage at CR1404 and R1402 is still in-spec at 55.6 volts.


Checking other transistors in the low-side of the inverter showed similar discrepancies to what their expected bias voltages are. Keep in mind I don't have a second scope (yet) to view the waveforms. Using my DMM, I checked Q1418 and found the bias levels near (not exact though) expected values -- emitter was ground, collector was at 21.0 volts (should be 24 I believe but this is close), and the base was at 0.22 volts (seems low though). Checking with the AC setting on my meter showed a ripple at 38 kHz on the collector and base, with voltages of 285mv and 71mv respectively.


Without a scope I can't really place much confidence in these AC readings, but given the low output voltage on the high side of the inverter, I think it's safe to assume something is far off. Lower than expected amplitude and frequency on the low side could cause this.


My plan is to pull Q1404 and test it, and to check the other transistors in that circuit as well. I also may have a second scope tomorrow to test with. Do you guys think seeing +146 volts on the 120volt unregulated DC rail is any issue? Could there be some strange "additive" short from another rail causing the increase? In case you're wondering, the AC line selector is correctly set and the line voltage was 116 volts AC RMS last night.


Your suggestions and comments have been very helpful, thanks again and they are always appreciated!


--Elliott

Do you guys think seeing +146 volts on the 120volt unregulated DC rail is any issue?

+146 V is absolutely acceptable on the 120 V unregulated line. Values above +150 V have been reported without anything being wrong.

Raymond